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ABOUT THE CAPACITY OF A DS S P R E A D SPECT RL I ;W CDM 1 CELLULAR SYSTEM

In Direct Sequence Spread Spectrum (IX44 Cellular. each user is provided with an individualand distinctive spread spectrum pseudo-noise (I,V) code. As in all A-trt s pwud i~zg techniques, thenarrow band message signal is multiplied by the spread spectrum pseudo-noise PA\; and thentransmitted. Because fN codes are uncorrelated with each other, many users can transmit at thesame time and in the same radio bandwidth, then we- have universal frequency reuse. This have

the consequence that there is no need for a mobile to change its frequency when moving into another

cell, so we have also Soft Handoff.Let-s remember that by perfect -47 (~.ilzl[o~ncrii~ POHXY Corzt~-~~~, we assume that we have theideal case where the signals received at the base station ofaceil from all the mobile units withinremains at the same power level, independent of the movement, propagation path loss and locationofthe mobiles. So by this fact we eliminate the near-far interference problem. This ideal case giveus an optimum for the capacity of a (IM44 cellular system. This capacity can be greater than othermultiple access techniques because contrary to them, (ZlMll users have the same radio bandwidthat the same time, this causing unfortunately high cochannel interference which must be fought bqstrong power control.

Before entering in details, lets say that we are in this work concern only by the capacity of theuplink(, mobile to base ) called also reverse link. We keep our attention to the uplink because itis the one which is the most affected by the near-far effect, a mobile user which try to reach the

base station see other users as source of interferences ( since all use the same bandwidth j ; thesignal from the nearest mobile to the base dominating the ones of others. Then in the uplink, thenear-far problem is important because to much inter&rers are involved contrary to the downlink(base to mobile) where the base station because of his tixed and elevated position have a access withless opposition to a mobile. So, we need strong power control in the uptink if we want to havethe great capacity that CD& i 4 can offer us.

We will discuss the case of imperfect AK: later, we firstly consider that we are in a perfectAPC case and in multiple cells system in which we choose a cell and a inside mobile user ofreference.

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The correction brought by P against the power control error will allow a better (accurate ) receivedsigal. References [7] and [8] give the complex expression of the conjoint probability densityfunction of the amplitude of the received signal and the power control error.

Before finishing, lets say that because users share the same radio frequency band at the sametime, the capacity of the II,\ Sprru~fSprct~-zdrzz (~ZXl! can be much greater then other multiple accesstechnologies? but is limited by interferences. IfI.lNil~ and FDhL4 are bandwidth limited, in ~DAL4i+-e need strong power control to solve the interferences problems and at the same time take thezd\.antnge of the possible great capacity. In our days, with the advances in signal processing,electronic, knowledges in waves propagation, we can have accurate power control techniques. Thishas helped the actual grow of (~O:WI.KEFERENCES